The present- invention relates to image displays, and more particularly to pulsed current control in image displays.
Flat panel displays are widely used in a variety of applications, including computer displays. One type of device well-suited for such applications is the field emission display. Field emission displays typically include a generally planar substrate having an array of projecting emitters. In many cases, the emitters are conical projections integral to the substrate. Typically, the emitters are grouped into emitter sets where the bases of the emitters in each set are commonly connected.
A conductive extraction grid is positioned above the emitters and driven with a voltage of about 30V-120V. The emitters are then selectively activated by providing a current path from the bases to the ground. Providing a current path to ground allows electrons to be drawn from the emitters by the extraction grid voltage. If the voltage differential between the emitters and extraction grid is sufficiently high, the resulting electric field causes the emitters to emit electrons.
The field emission display also includes a display screen mounted adjacent the substrate. The display screen is formed from a glass plate coated with a transparent conductive material to form an anode biased to about 1kV-2kV. A cathodoluminescent layer covers the exposed surface of the anode. The emitted electrons are attracted by the anode and strike the cathodoluminescent layer, causing the cathodoluminescent layer to emit light at the impact site. The emitted light then passes through the anode and the glass plate where it is visible to a viewer.
The brightness of the light produced in response to the emitted electrons depends, in part, upon the number of electrons striking the cathodoluminescent layer in a given interval. The number of emitted electrons depends in turn upon the magnitude of current flow to the emitters. The brightness of each area can thus be controlled by controlling the current flow to the respective emitter. The light emitted from each of the areas thus becomes all or part of a picture element or xe2x80x9cpixel.xe2x80x9d
In a typical analog voltage control approach, current flow to the emitters is controlled by controlling the voltage applied to either the emitters or the extraction grid to produce a selected voltage differential between the emitters and the extraction grid. The electric field intensity between the emitters and the extraction grid is the voltage differential divided by the distance between the emitters and the extraction grid. The magnitude of the current to the emitters then corresponds to the intensity of the electric field.
As is known, analog voltage control approaches can be relatively complex to implement, especially in displays that typically receive digital image signals, such as displays intended for laptop computers as well as large xe2x80x9cpassive matrixxe2x80x9d displays. A passive matrix field emission display is a display in which a single driving circuit is provided for a group of emitters, such as a row or column of emitters. In contrast, in an xe2x80x9cactive matrixxe2x80x9d field emission display, a respective driving circuit is provided for each emitter or group of emitters that are in the same pixel of the display.
Analog voltages can also be difficult to control precisely due to variations in component values caused by temperature, age, or other conditions. In large arrays, variations in transistors, emitters or the extraction grid can result in non-uniform display characteristics or otherwise detrimentally affect performance.
One approach to reducing this problem employs pulse-width modulation. In this approach, the image signal is converted to a pulse-width modulated signal where the pulse width is determined by the value of the image signal. Then, the emitter is activated by grounding the emitter during an xe2x80x9cONxe2x80x9d time corresponding to the width of the pulse. Pulse width modulation typically requires conversion of the input signal from an analog signal to a pulse width modulated signal. Typical techniques for such conversion may introduce errors and increase the complexity of the driving circuitry. Moreover, typical implementations of pulse width modulation require precise control of timing.
In accordance with the invention, a control circuit modulates the number of times that an emitter or group of emitters in the same pixel emits light during an activation interval to control the intensity of the pixel. Each pulse of a clocking signal couples the emitter or group of emitters to a voltage having a value that causes the emitter or group of emitters to emit electrons. The number of electrons emitted in a selected activation interval is controlled by controlling the number of such pulses during the activation interval.
The number of pulses of the clocking signal during each activation interval is determined in response to an image signal. In one embodiment where the image signal is a digital signal, the display includes a plurality of clock sources, each producing a respective set of pulses. Pulses from each clock source are selectively passed or blocked based upon the state of a respective bit of the digital image signal. Then, all of the passed pulses are accumulated to form the clocking signal.
In another embodiment, the image signal is decoded to produce a binary number. At the beginning of each activation interval, a counter begins decrementing responsive to a continuous clock signal. A comparing circuit compares the count to the binary number and, when the count matches the binary number, the comparing circuit outputs a disable pulse. From the beginning of the activation interval until the disable pulse arrives, a pulse source outputs a series of equally spaced pulses of the clocking signal. Consequently, the pulse source outputs a number of clocking signal pulses corresponding to the binary number.
The pulse number modulation circuit and method is preferably used in a passive field emission display such as a display in which a respective driving circuit is provided for the emitters or groups of emitters in each column of the display, and the extraction grids in each row are coupled together. However, the pulse number modulation circuit and method may also be used in an active field emission display in which a respective driving circuit is provided for each emitter or group of emitters in the same pixel.